This invention relates to a new process for preparing 4-piperidone spiroketal compounds in excellent yield and purity.
Hindered piperidine alcohol compounds having Formula I, which can be named 2,2,6,6-tetramethyl-4-piperidone spiroketal carbinol compounds, or more systematically 9-aza-3-hydroxymethyl-3-alkyl-8,8,10,10-tetramethyl-1,5-dioxaspiro(5,5) undecanes, ##STR3## have been disclosed by K. Murayama in U.S. Pat. No. 3,899,464 of Aug. 12, 1975 as stabilizers able to protect synthetic polymers and plastics against the harmful effects of exposure to ultraviolet radiation and heat. Compounds of Formula I have also been disclosed to be valuable synthetic intermediates for the preparation of even better stabilizers by reaction of the compounds at the alcoholic hydroxyl group to form various derivatives. Outstandingly effective stabilizers among these derivatives are certain organic phosphite esters disclosed by M. Minagawa et al in U.S. Pat. No. 4,096,114 of June 20, 1978; hydroxyaliphatic dicarboxylic and tricarboxylic acid esters disclosed by M. Minagawa et al in U.S. Pat. No. 4,105,625 of Aug. 8, 1978; diol bis-carbonate esters disclosed by M. Minagawa et al in U.S. Pat. No. 4,115,476 of Sept. 19, 1978; butane-and butenetricarboxylic acid esters diclosed by M. Minagawa et al in U.S. Pat. No. 4,116,927 of Sept. 26, 1978; heterocyclic carboxylic acid esters disclosed by M. Minagawa et al in U.S. Pat. No. 4,118,369 of Oct. 3, 1978; and aliphatic tetracarboxylic acid esters disclosed by M. Minagawa et al in U.S. Pat. No. 4,136,081 of Jan. 23, 1979.
Compounds of Formula I have hitherto been prepared as disclosed by Murayama by the reaction of a tetraalkyl-4-piperidone with a trimethylolalkane in the presence of an acid catalyst. However, the procedure has required long reaction times and the product was impure; in Murayama's description of this process no product yields are given.
Ketal derivatives of 4-piperidones have been known for some time, but no exchange reactions of 4-piperidone ketals with alcohols have been reported. P. Brookes and J. Walker in J. Chem. Soc. 1957, pages 3172-3175, disclosed the ready conversion of 1-n-butyl-4-piperidone hydrochloride to 1-n-butyl-4-piperidone dimethyl and diethyl ketal hydrochlorides by warming with methanol or ethanol respectively, the similar conversion of 1-benzyl-4-piperidone hydrochloride to 1-benzyl-4-piperidone diethyl ketal hydrochloride by warming with ethanol, and the ready conversion of 1-benzyl-4-piperidone diethyl ketal hydrochloride to 1-benzyl-4-piperidone hydrochloride by warming with hydrochloric acid, but noted that 1-n-butyl-4-piperidone dimethyl ketal hydrochloride was recovered unchanged after warming with ethanol. Thus no exchange of one alcohol for another in a piperidone ketal occurred under the conditions used by Brookes and Walker.